This invention relates to a wrapping machine for wrapping items in stretchable films.
Many years have passed since the introduction of practical wrapping machines wherein containers or trays made of resin materials such as foamed styrene are supplied with various articles and then wrapped individually in transparent stretchable film. During this period a variety of wrapping machines have been proposed and improved upon.
Wrapping machines for wrapping articles in stretchable film on a tray-by-tray basis are equipped with various means for avoiding a wasteful wrapping operation while at the same time improving the appearance of the wrapped package. In particular, to obtain a wrapped package having an attractive appearance, means of paramount importance are a film folding mechanism for folding the stretchable film under the tray bearing the items to be packaged, a control mechanism for adjustably controlling the timing at which various operating units constituting the wrapping machine are actuated, and a film tension adjusting mechanism for adjusting the tension of the stretchable film stretched over the items on the tray.
Almost all of the conventional film folding mechanisms for folding the stretchable film under the trays include such elements as left and right folding plates and a front folding plate. These elements are mechanically coupled with a source of drive via a mechanism such as a drank link and are adapted to be actuated by power transmitted from the drive source. An example of such a wrapping machine is disclosed in the specification of Japanese Patent Application Laid-Open No. 58-160230. The disclosed wrapping machine includes left and right folding plates which perform arcuate motion, each folding plate being provided with a guide groove, and a front folding plate having rollers arranged in the guide grooves so that the front folding plate operates in tandem with the left and right folding plates.
The above-described prior-art folding mechanism has a number of drawbacks. Specifically, since the stretchable film covering the items on a tray is folded under the tray by being squeezed and pressed by the left and right folding plates and the rollers of the front folding plate, the film develops a large number of creases and wrinkles that detract from the appearance of the final package. Moreover, there are cases where the items being wrapped become jammed in the folding mechanism. Since the various operating elements of the conventional folding mechanism are mechanically coupled with the drive source via the mechanism such as the crank link, the drive source must be operated by hand to move the operating elements when such jamming occurs in order to permit removal of the items that have been turned over or that have becomed jammed in the machine. These operating elements must then be returned to their original positions following removal of the items, after which the machine is restarted. Thus, restoring the machine to operation is a difficult and time-consuming task.
Since the left and right folding plates and the front folding plate are mechanically connected, the timing at which each folding plate is started cannot be adjusted independently of the others. By way of example, assume that the tray which receives the articles to be wrapped is comparatively small. In such case, the opening formed by the folding plates will be too large in comparison with the size of the tray, with the result that opposing edges of the film cannot be folded under the tray in reliable fashion even though the left and right folding plates are actuated. The results will be the same even if the film is cut to a small length.
The conventional control mechanism for controlling the operation timing of the elements constituting the wrapping machine relies upon mechanical means composed mainly of a combination of various cams. In addition, analog-type means such as a volume resistor are used to set the cut length of film and well as the operation timing of the left, right and front folding plates.
The method of adjusting the operation timing of the various wrapping machine elements by relying upon the cam mechanism involves difficulties in terms of on-site assembly, adjustment and maintenance. These operations cannot be performed well unless done by one having special skills. Another disadvantage is that since the wrapping machines manufactured cannot provide a uniform mechanical timing from one machine to another owing to the machining precision of the component parts and errors in assembly, mechanical adjustments must be made on all such occasions. The result is a decline in operation efficiency. Problems are also encountered in the analog means such as the volume resistor for setting the cut length of the film and the operation timing of the folding plates. Specifically, not only is it difficult to make the adjustments when the wrapping machine is assembled, but a difficult readjustment is necessary whenever the machine becomes maladjusted.
The conventional means for adjusting the tension of the stretchable film convering the articles in the tray includes a clamping mechanism and a lifting mechanism. When the stretchable film cut to predetermined dimensions is extended over substantially the center of the wrapping machine, the edges of the stretchable film are held fast by the clamping mechanism and the articles to be wrapped are raised into stretching engagement with the stretchable film by the lifting mechanism, whereby the stretchable film covering the articles is stretched and brought into intimate contact with the articles. When the edges of the stretchable film are subsequently folded under the tray by the folding mechanism, the clamping mechanism is actuated at a predetermined timing to release its hold on the edges of the stretchable film. This leaves the stretchable film of the package in the stretched state. However, since the stretched state of the film wrapping the package is influenced by the dimensions of the film as well as such dimensions as the length, width and height of the articles to be wrapped, the final wrapped packages produced by the wrapping machine using the conventional tension adjusting method lack a uniform appearance due to the size of the articles wrapped.